The present disclosure relates to a semiconductor optical device.
A structure called “multiquantum well structure”, the structure having plural quantum well layers (light emitting layers) layered over one another, has been widely used as an active layer of a semiconductor optical device, such as a semiconductor laser. As a technique for broadening a gain bandwidth of a semiconductor laser, layering plural quantum well layers over one another, the quantum well layers having slightly different thicknesses, has been known.
Further, as a technique for broadening a gain bandwidth of a semiconductor optical amplifier, arranging two quantum well layers having different gain peak wavelengths in a direction of propagation has been disclosed in Japanese Patent No. 4005705.
However, in both of: the case where multiple quantum well layers having slightly different thicknesses are layered over one another; and the case where quantum well layers having different gain peak wavelengths are arranged in the direction of propagation; when a difference among/between the respective gain peak wavelengths is increased, the gain is reduced at the wavelength between the peak wavelengths and flat gain characteristics are unable to be obtained. Therefore, the difference among/between the peak wavelengths of the respective quantum well layers needs to be made small.
However, when the difference among/between the peak wavelengths of the respective quantum well layers is made small, in broadening the flat gain bandwidth, there is a problem that multiple quantum well layers having different thicknesses will be required.
Further, even if multiple quantum well layers having peak wavelengths over a broad range and having different thicknesses are layered over one another, since quantum well layers having gain at a long wavelength side act as an absorber layer for light on a short wavelength side, it is difficult to broaden the gain bandwidth substantially.
The present disclosure has been made in view of the above, and an object thereof is to provide a semiconductor optical device having a broad gain bandwidth while having a simple configuration.